1. Field of the Invention
The present invention relates to a heating fixer and an image forming apparatus, and more particularly to a heating fixer melting a toner adhering to a recording medium upon application of heat to fix the toner thereon and to an image forming apparatus including the fixer, such as a copier, a printer, and a facsimile machine.
2. Discussion of the Background
Image forming apparatuses such as electrophotographic copiers, printers and facsimile machines use heating fixers fixing toner images transferred onto recording media such as paper upon application of pressure and heat to melt the toner images. A sheet of paper on which is formed an unfixed toner image is passed through a nip formed by a fixing roller heated by a heater and a pressure roller to fix the toner image on the paper.
It is essential for such heating fixers to apply an optimum amount of heat, which varies depending on the type of paper used and its thickness. For example, there is known a method of changing a temperature of the fixing roller to change an amount of heat applied to a paper. However, it takes time to control the temperature of the fixing roller, which occasionally causes a downtime. Other methods involve changing a pressure applied by the pressure roller to the fixing roller and a width of the nip to control an amount of heat when a sheet of paper passes through the nip.
The heating fixer having a changeable nip width has, e.g., a configuration like that shown in FIG. 1A, in which a biasing member 108 as a pressurizer contacts a pressure roller (pressure member) 105 with an unillustrated fixing roller (fixing member) upon application of pressure, and the pressurizer controls the pressure to the pressure roller 105 by rotation of a cam 101, indicated by a two-headed arrow.
As FIG. 1B shows, a blocking filler 102 rotatable in the direction of an arrow with the cam 101 blocks light received by a transmission sensor 103 to control the cam 101. For example, after the blocking filler 102 blocks light received by a transmission sensor 103, the blocking filler 102 stops at a home position (hereinafter referred to as “H.P.”) shown on the right after a specific number of rotations. Then, the blocking filler 102 rotates at a specific angle according to the type of paper and its thickness to a pressure position shown on the left.
However, every time the type of paper or its thickness is changed, the blocking filler 102 needs to return to the H.P., which takes time.
It is possible to shorten the time if the nip width can be adjusted by setting a rotational direction and a minimum rotational amount of the cam needed while keeping the nip width information at the time without returning the blocking filler to the H.P. when the type of paper or its thickness is changed.
However, when such a control is repeated, it is possible that the movement of the blocking filler to the H.P. is gradually shifted due to gear backlash from a gear driving the cam.
For these reasons, a need exists for a heating fixer capable of stopping the cam at a desired position even when repeating operation without transferring the cam to the H.P., and quickly and precisely optimizing a nip width according to the type of paper used and its thickness.